The present invention relates to manufacture of hollow glass microspheres, and more particularly to manufacture of "high-silica" glass microspheres from a gel and to the resulting product.
It has been found desirable in the art of manufacturing hollow glass microspheres or shells to provide microspheres constructed essentially of a single oxide, preferably silica. Such single oxide shells avoid many potential problems associated with multiple component glass shells, such as inhomogeneities and phase separation. Essentially pure silica shells are particularly desirable for their high strength and resistance to devitrification. Moreover, essentially pure silica microspheres are considered desirable for use as inertial confinement fusion targets because of controllable and predictable gas permeation characteristics, and because of the high softening temperatures characteristic of silica. However, the high softening temperature of pure silica (1667.degree. C.) presents a major difficulty in attempts to manufacture pure or essentially pure silica microspheres.
In the manufacture of silica microspheres from a glass gel, boron is often added to the gel for lowering the viscosity and, therefore, the softening temperature of the dried and crushed gel. However, the resulting glass microspheres possess a substantial boron concentration. Concerning the manufacture of hollow glass microspheres from gels and a gel powder, reference may be had to the following art: the U.S. patents to Veatch et al Nos. 3,030,215, Beck et al 3,365,315, and Budrick et al 4,017,290 and 4,021,253; Souers et al, "Fabrication of the Glass Microballoon Laser Target", UCRL-51609, Sept. 26, 1974; and 1977 Annual Report on Laser Fusion Research, KMS Fusion, Inc., pages 1-12 to 1-15.
A general object of the present invention is to provide a method of manufacturing hollow glass microspheres of essentially pure silica glass. More specifically, an object of the invention is to provide a method of manufacturing hollow glass microspheres composed of at least 99% silica.
Another object of the invention is to provide a gel powder suitable for manufacture of essentially pure silica glass microspheres. A further and related object of the invention is to provide a method of gel formation suitable for manufacture of such frit.
Yet another object of the invention is to provide hollow glass microspheres composed of at least 99% silica.